A mixed-breed dog, also called a mutt, mongrel, tyke, cur, bitzer, feist or random-bred dog, is a dog that has characteristics of two or more types of breeds, or is a descendant of feral or pariah dog populations.

This term, along with the term purebred dog stems from a fundamental misunderstanding of the genetics and misinterpretation of Darwinian theory, and should be viewed with the same skepticism as we view the concept of pure races in human populations. The term crossbreed refers to puppies produced by the breeding technique of breeding known dogs of two different purebred dog breeds for some specific reason.

The term mutt generally refers to a dog of unknown descent. Dogs interbreed freely, except where extreme variations in size exist, so mixed-breed dogs vary in size, shape, and color, making them hard to classify physically.